Alternating-piston compressors generate pressure by compressing the gas inside a cylinder through axial movement of the piston, so that the gas on the low-pressure side, i.e., the side of suction or evaporation pressure, goes into the cylinder through the suction valve. The gas is then compressed inside the cylinder by piston movement and after having been compressed it comes out of cha cylinder through the discharge valve to the high-pressure side, i.e., the side of discharge pressure or condensation.
Especially for resonant linear compressors, the piston is actuated by a linear actuator, which is formed by a support and magnets that can be actuated by one or more coils. The linear compressor further comprises one or more springs, which connect the moveable part (piston, support magnets) to the fixed part, the latter being formed by the cylinder, stator, coil head and frame. The moveable parts and the springs form the resonant assembly of the compressor.
Such a resonant assembly, driven by a linear motor, has the function of developing a linear alternating movement, causing the movement of the piston inside the cylinder to exert a compression action of the gas admitted through the suction valve as far as it can be discharged through the discharge valve into the high-pressure side.
The operation amplitude of the linear compressor is regulated by balance of the power generated by the motor, with the power consumed by the compression mechanism, besides the losses generated in this process. In order to achieve maximum thermodynamic efficiency and maximum cooling capacity, the maximum displacement of the piston should come as close as possible to the end of stroke, thus reducing the dead gas volume in the compression process.
In order to make this process feasible, it is necessary for the piston stroke to be known in great accuracy, so as to prevent the risk of piston impact at the end of the stroke on the equipment head. This impact might generate an acoustic noise, loss of efficiency of the apparatus or even breakdown of the compressor.
Thus, the greater the error in estimating/measuring the piston power the greater the safety coefficient required between maximum displacement and the stroke end, to operate the compressor in security, which leads to loss of performance of the product.
On the other hand, if it is necessary to reduce the cooling capacity of the compressor due to less need to use the cooling system, it is possible to reduce the maximum piston-operation stroke, thus reducing the power supplied by the compressor, and so it is possible to control the cooling capacity of the compressor, thereby obtaining a varying capacity. An important additional characteristic in operating resonant linear compressors is the actuation frequency thereof. A few prior techniques show that actuating the compressor at its resonance frequency causes the equipment to work at its maximum efficiency.
However, such techniques usually make use of position and/or velocity sensors for operating the system, which increases the final costs of the product considerably.
Hereinafter one makes a brief description of prior-art solutions employed today for knowing the compressor-piston stroke. The documents cited below make use of position sensors, for example the Brazilian case PI 0001404-4. This document further has the disadvantages of difficulty of isolation and electric contact noise.
Document PI 0203724-6 relates to a fluid pump and a fluid-transfer plate, such elements being particularly applicable to linear compressors for detecting the position of the respective piston and preventing the latter from colliding with the fluid-transfer plate or the valve plate upon variations in the operation conditions of the compressor, or even variations in the feed voltage. Such a technique employs en inductive sensor mounted on the valve plate, in order to measure the piston/plate distance directly at the piston top. This is a high-precision solution, but it requires space for installing the sensor on the valve plate and is more expensive, besides requiring calibration.
Other prior-art solutions, like those described in documents U.S. Pat. No. 5,897,296, JP 1,336,661 and U.S. Pat. No. 5,897,269, make use of a position sensor. Therefore, such applications exhibit greater implementation and/or maintenance complexity, besides a higher cost. It should be further pointed out that, in these latter cases, there is a need for a greater number of wires and external connections to the compressor, which makes difficult the use thereof in environments of great variation in temperature and pressure.
On the other hand, a few prior-art technique that do not use position sensors, like documents U.S. Pat. Nos. 5,342,176, 5,496,153, 4,642,547 and U.S. Pat. No. 6,176,683, besides KR 96-79125, KR 96-15062, WO00079671 and WO03044365, do not exhibit good accuracy or operation stability, for which reason it is necessary to employ other types of sensors, such as temperature meters or accelerometer for detecting impact, besides a more expensive sizing for the compressor in view of the requirements of performances for its correct functioning.
On the basis of the foregoing, the present invention foresees a system and a method for controlling the pistons of a resonant linear compressor, especially designed for actuating the compressor at its maximum efficiency, without using sensors to measure mechanical magnitudes or variables.